Electro-optical devices from banana-shaped liquid crystals

ABSTRACT

A liquid crystal device comprising tilted smectic phases of banana-shaped liquid crystal molecules is disclosed. A method for fabricating a light modulating device is also disclosed. The method comprises the steps of providing a pair of substrates with a cell gap therebetween and permanently disposing at least one banana-shaped liquid crystal material into said cell gap. The present invention also provides a method of generating an image, comprising providing a pair of substrates with a cell gap therebetween, providing transparent electrodes on each of the substrates adjacent to the cell gap, disposing at least one banana-shaped liquid crystal material into the cell gap; and applying an electric field across the electrodes. The tilted smectic phases of banana-shaped liquid crystal may be in either a racemic or a chiral state. The application of a sufficiently high electric field transitions the banana-shaped liquid crystal material between the racemic and chiral states, and both the racemic and the chiral states are stable in the absence of an electric field.

[0001] This application claims benefit of pending U.S. ProvisionalApplication No. 60/243,371 filed on Oct. 26, 2000.

GOVERNMENT RIGHTS

[0002] The United States Government has a paid-up license in thisinvention and may have the right in limited circumstances to require thepatent owner to license others on reasonable terms as provided for bythe terms of Grant DMR8920147-14, awarded by the National ScienceFoundation.

TECHNICAL FIELD

[0003] The present invention resides in the art of electro-opticalliquid crystal devices made with banana-shaped molecules. These devices,which may be used for electro-optical switching and electro-opticalstorage using liquid crystal devices, utilizes the tilted smectic phaseof banana-shaped molecules.

BACKGROUND OF THE INVENTION

[0004] Liquid crystal materials are materials which occupy anintermediate state between crystalline solid materials and isotropicliquid materials. Liquid crystal materials, while exhibiting anorientational order, do not typically exhibit a positional order. Theunique properties of liquid crystal materials have enabled their use ina variety of display applications. Among the useful properties of liquidcrystal materials in display applications are the reflection andrefraction of light by the liquid crystal (LC) and the ability of theuser to influence these properties. These properties are governed by theorientation of the molecules which comprise the liquid crystal. Theorientation of individual molecules often determines the behavior oflayers and phases of these molecules.

[0005] The lack of mirror symmetry of individual molecules is describedas the chirality or “handedness” of the molecule. Many liquid crystalphases are chiral due to the introduction of chirality of the same signat the molecular level. Examples of these types of chiral liquid crystalphases include cholesteric, blue, Twist Grain Boundary (TGB) and smecticC* phases. Due to the long-range orientation order of liquid crystallinephases, and the chirality of the molecules, a spontaneous twist occursin a micrometer range. The chirality transfers from a molecular tomesoscopic range, and the phase becomes chiral.

[0006] Two molecules that are identical in composition yet are mirrorimages of each other are described as having opposite chirality. This isgenerally expressed as the molecules being left-handed or right-handeddepending on their particular orientation. Liquid crystal moleculeshaving the same chemical formula but opposite chirality will behave inoptically similar, but oppositely directed ways.

[0007] Scattering type devices are very well known in liquid crystaldisplays. Two known types are polymer dispersed liquid crystals (PDLC),and polymer network containing liquid crystals (PNLC). Liquid crystalpolymer dispersions form a broad class of materials in which the weightconcentration of polymer ranges from 2% to 90%, depending on theapplication and type of polymer used. Dispersions, wherein the liquidcrystal forms nearly spherical droplets randomly distributed throughouta polymer matrix, and the polymer concentration is 20% or more, arenormally referred to as polymer dispersed liquid crystals (PDLC).Normally, PDLCs are light scattering in the “off” state and transparentin the “on” state. It is also possible to make reverse mode PDLCs. Thedisplay modes, however, cannot be interchanged.

[0008] PNLCs are formed by photopolymerization of a mixture containingless than 10% of a reactive monomer in an aligned liquid crystal host,such as a nematic, ferroelectric, or cholesteric phase liquid crystalmaterial. The alignment may be assisted by surface alignment layers orby external fields. The polymerization induces phase separation of aninitially homogeneous mixture. The morphology of the polymer networkdepends on the orientational order of the liquid crystal, properties ofthe monomer, and the presence of external aligning fields and/orconventional alignment layers applied to the cell surfaces. Normally,PNLCs work as reverse mode PDLCs. It is also possible to make PNLCs thatare opaque at zero fields. Once made, however, the display modes cannotbe interchanged. The switching times in PDLCs and PNLCs are typicallyover a millisecond, which is not optimal for most video applications.Moreover, the viewing angle and transmittance of the clear state arelimited.

[0009] In light of the foregoing, it is evident that there is a need inthe art for an electro-optical liquid crystal device which has fasterswitching times, a wider viewing angle, and improved transmittance ofthe clear state. It would be additionally advantageous if the liquidcrystal device contained electro-optical storage functionality.

BRIEF SUMMARY OF THE INVENTION

[0010] In light of the foregoing, it is a first aspect of the presentinvention to provide a liquid crystal display device having fasterswitching times, a wider viewing angle, and improved transmittance ofthe clear state.

[0011] It is a further aspect of the present invention to provide aliquid crystal device capable of electro-optical storage functionality.

[0012] The aspects of the invention are achieved by a liquid crystaldisplay device comprising the tilted smectic phase of banana-shapedliquid crystal molecules. In one embodiment of the invention, a cell isprovided containing the racemic state of banana-shaped liquid crystalmolecules. As used herein, a banana-shaped LC domain is referred to asracemic if the chirality of the layers generally alternates from onelayer to the next. The cell is opaque at zero field, and clear when anelectric field of sufficient magnitude is applied. The clear state isclear in any direction, hence these cells have a very wide viewingangle. This is a tilt separation mode liquid crystal device (TSM-LCD).

[0013] In a second embodiment of the invention, a cell is providedcontaining the chiral state of the banana-shaped molecules. As usedherein with respect to domains of banana-shaped liquid crystal, the termchiral indicates that adjacent layers of liquid crystal generally havethe same chirality or handedness. A cell with a chiral layer arrangementis clear under zero fields, and becomes opaque when an electric field ofsufficient magnitude is applied. The switching time is more than anorder of magnitude faster than the switching time for PDLCs. This typeof arrangement is used in a chiral separation mode liquid crystal device(CSM-LCD).

[0014] In a third embodiment, a cell is provided containing racemic andchiral state banana-shaped molecules. By applying an electric field, theracemic state is converted to the chiral state, or the chiral state isconverted to the racemic state. Both states are stable at zero field. Inother words, the LC material may be driven to either state with anapplied electric field, and when the field is removed, the state remainsindefinitely. The racemic state is opaque and the chiral state is clear.This cell is suitable as an electro-optical storage device or anelectro-optical switching device.

[0015] The present invention also provides a method for fabricating alight modulating device, the method comprising the steps of providing apair of substrates with a cell gap therebetween, and permanentlydisposing at least one banana-shaped liquid crystal material into thecell gap.

[0016] The present invention also provides a method of generating animage, comprising providing a pair of substrates with a cell gaptherebetween, providing transparent electrodes on each of the substratesadjacent to the cell gap, permanently disposing at least onebanana-shaped liquid crystal material into the cell gap; and applying anelectric field across the electrodes to obtain a desired optical state.

[0017] These and other aspects of the present invention, as well as theadvantages thereof over existing prior art forms, which will becomeapparent form the description to follow, are accomplished by theimprovements hereinafter described and claimed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0018] For a complete understanding of the objects, techniques andstructure of the invention, reference should be made to the followingdetailed description and accompanying drawings, wherein:

[0019]FIG. 1A is the chemical structure of4-chloro-1,3-phenylenesbis[4-(4-14 alkyloxyphenyliminomethyl) benzoate,a banana-shaped LC with one central phenyl4 ring;

[0020]FIG. 1B is the chemical structure of3-chloro-3,4′biphenylenebis[4(tetradecylphenyliminomethyl) benzoate, abanana-shaped LC with a 3,4′dihydroxybiphenyl central core;

[0021]FIG. 2A is a representation of a left hand and a left-handedbanana-shaped LC;

[0022]FIG. 2B is a representation of a right hand and a right-handedbanana-shaped LC;

[0023]FIG. 3A is a schematic orthogonal view of the racemic B2 phase ofachiral banana-shaped molecules in an antiferroelectric (AFE) state;

[0024]FIG. 3B is a schematic orthogonal view of the racemic B2 phase ofachiral banana-shaped molecules in a ferroelectric (FE) state;

[0025]FIG. 4A is a schematic orthogonal view of the chiral B2 phase ofachiral banana-shaped molecules in antiferroelectric (AFE) states;

[0026]FIG. 4B is a schematic orthogonal view of the chiral B2 phase ofachiral banana-shaped molecules in ferroelectric (FE) states;

[0027]FIG. 5 is an enlarged, partial cross-sectional, schematic view ofa light modulating device according to the present invention.

[0028]FIG. 6 is a graphical representation of the electric fielddependence of the transmitted light intensity of type 1 cell (d=4-μm,T=70° C., λ=450 nm);

[0029]FIG. 7 is a photomicrograph of use of the B2 banana phase as anelectrically switchable light shutter, utilizing a 10-μm EHC cell with 1cm² active area of material #1 at room temperature;

[0030]FIG. 8 is a photomicrograph of a cell according to the presentinvention at room temperature in reflection at zero voltage and at 40V;

[0031]FIG. 9A is a photomicrograph of textures of a cell according tothe present invention in transmission mode;

[0032]FIG. 9B is a photomicrograph of textures of a 4-μm thick cell ofmaterial #1 in transmission mode;

[0033]FIG. 10 is a graphical representation of the transmission spectraof a 4 μm thick cell of material #1;

[0034]FIG. 11A is a graphical representation of the voltage dependenceof the time for switching between transparent and scattering states formaterial #1;

[0035]FIG. 11B is a graphical representation of the temperaturedependence of the time for switching between transparent and scatteringstates for material #1;

[0036]FIG. 12A is a photomicrograph of the texture of a cell of materialB14 with 1.5% racemic dopant at 0 V;

[0037]FIG. 12B is a photomicrograph of the texture of a cell of materialB14 with 1.5% racemic dopant at 30 V;

[0038]FIG. 12C is a photomicrograph of the texture of a cell of materialB14 with 1.5% (S)-enantiomer dopant at 0 V; and

[0039]FIG. 12D is a photomicrograph of the texture of a cell of materialB14 with 1.5% (S)-enantiomer dopant at 30 V.

DETAILED DESCRIPTION OF THE INVENTION

[0040] Tilted smectic phases of achiral banana-shaped liquid crystalmolecules have been observed. Banana-shaped or “bent core” liquidcrystal molecules are individually symmetric and therefore have nochirality individually. Nonetheless, it has been observed that they canarrange in layers that exhibit chirality. It is believed that thisbehavior is due to spontaneous symmetry breaking which has been observedin tilted smectic phases of bent-core, banana-shaped molecules. Thisphase, sometimes called the B2 phase, is a 2-dimensional fluid. Themolecules adopt a uniform tilt relative to the layer polarization, whichis determined by the two-fold symmetry axis. Due to the tilt and thepolar packing of the molecules, the layers have no reflection symmetryand are therefore chiral. The present invention utilizes banana-shapedLCs to construct liquid crystal devices. The resulting devices may bemanipulated such that they may be reversibly changed from a lightscattering state to a transparent state and vice versa. These devicesinclude, but are not limited to computer displays, computer monitors,signs, shutters, gratings, optical devices or any other device thattransmits, reflects or modulates light of any wavelength. Thereversibility between states is preferably performed with application ofelectric fields, but could also be accomplished thermally ormechanically.

[0041] The banana-shaped LC materials of the present invention may bedescribed with reference to FIG. 1A and FIG. 1B. FIG. 1A shows thechemical structure of the banana-shaped LC material4-chloro-1,3-phenylinabis-1,3-phenylenebis[4-(4-tetradecylpheniyliminomethyl)benzoate. FIG. 1B shows the chemical structure of3-chloro-3,4′biphenylenebis[4-(tetradecylphenyliminomethyl) benzoate. Asshown in FIGS. 1A and 1B, the molecules have a “bent-core” or“banana-shaped” conformation. Other suitable banana-shaped LC materialsinclude those liquid crystals represented by formula I,

[0042] R₁, R₂, R₃, R₄ are independently hydrogen or a halogen, and R₅and R₆ are independently C₈-C₁₆ alkyl or C₈-C₁₆ alkoxy. This includes4-chloro-1,3-phenylinabis-1,3-phenylenebis[4-(4-tetradecoxyphenyliminomethyl)benzoate,as well as 1,3-phenylene bis [4-4 (4-n-alkylphenyliminomethyl)benzoates, 1, 3-phenylene bis [4-4(4-n-alkyloxyphenyliminomethyl)benzoates, and 1,3-phenylenebis[3-fluoro-(4-n-alkyloxyphenyliminomethyl)benzoates and halogenatedderivatives thereof.

[0043] It has been discovered that banana-shaped LC's can exhibit fourdifferent optical states which are antiferroelectric racemic,antiferroelectric chiral, ferroelectric racemic and ferroelectricchiral. As will be discussed in detail, these states are preferablyobtained by applying electric fields of different magnitude and/orfrequency. It is also believed that the magnitude and shape of theapplied electric field—for example, square or triangular—may be used toobtain a desired state. All of these states are obtained without theneed of alignment layers, although the use of alignment materials may bedesirable for some applications.

[0044] Prior to discussing each of the four states in detail, it isbelieved that the terms used to name the states should be defined andthat properties that characterize all of the states should also bedefined. These properties include: achiral, polar plane, tilt plane,layer, domain, synclinic and anticlinic. Chiral is a term used todescribe a molecule or group of molecules, for example, a layer ofliquid crystal molecules, which do not exhibit mirror symmetry. Achiral,on the other hand, is a term used to describe a molecule or group ofmolecules which exhibit mirror symmetry. Banana-shaped liquid crystalmolecules are smectic liquid crystals. That is, they arrange in layersof liquid crystal molecules with each layer having a particular averageorientational order. In the case of banana-shaped liquid crystals, eachlayer assumes particular polar and tilt directions. The tilt plane of alayer of banana-shaped LC is the plane which shows the tilt of themolecules within the layer relative to the layer normal. The polar planeis the plane which contains the layer normal and the layer polarization.

[0045] A group of layers exhibiting a particular pattern of propertiesis referred to herein as a domain or phase. A domain may be eitherracemic or chiral. A racemic domain is one in which the chirality or“handedness” of the layers alternates between left-handed andright-handed from layer to layer. A chiral domain, however, containslayers that have the same chirality.

[0046] A domain may also be described according to the tilt direction ofthe layers within the domain. A domain is synclinic if all the layerswithin the domain tilt in the same direction relative to the layernormal. A domain is anticlinic if the direction of the tilt of thelayers alternates from one layer to the next.

[0047] A domain may also be described by the presence or absence of anet polarization of the domain. A ferroelectric state is said to existif there is a net polarization of the domain. An antiferroelectric stateexists if the domain exhibits no net polarization.

[0048] Further attributes of these states and properties will becomeapparent as the description proceeds.

[0049] Eventhough the banana-shaped LCs of the present invention areachiral, they are capable of assembling to for chiral phases. Theorientation of a left-handed and a right handed banana-shaped LC,relative to the layer in which they are situated, is illustrated in FIG.2A and FIG. 2B, respectively. P is the layer polarization direction andn is the layer normal. The angle Θ is the angle formed between theaverage molecular axis of the layer and the layer normal. As shown inFIG. 2A, the average molecular axis of a left handed molecule isoriented clockwise from the layer normal where the layer polarization isperpendicular to the layer normal. This can be envisioned relative to aleft hand as illustrated in FIG. 2A. If the thumb of a left hand isenvisioned as pointing in the direction of layer polarization, thedirection of the curling of the fingers represents the direction of thedeviation of the molecular axis from the layer normal, forming angle Θ.Likewise, in FIG. 2B, the average molecular axis of a right handedmolecule is oriented counter-clockwise from the layer normal where thelayer polarization is perpendicular to the layer normal. In this case,when the thumb of a right hand is envisioned as pointing in thedirection of layer polarization, the direction of the curling of thefingers represents the direction of the deviation of the molecular axisfrom the layer normal, forming angle Θ.

[0050] The various arrangements of phases or states of banana-shaped LCmaterials according to the present invention are shown in FIGS. 3A, 3B,4A, and 4B. In these figures, the liquid crystal molecules are shownfrom both the tilt plane view and the polar plane view. The tilt planeview corresponds to the view as seen through a substrate of a liquidcrystal cell according to the present invention while the polar planeview is a view taken from a ninety degree rotation from the tilt planeview. Stated another way, the polar plane contains the layer normal andthe layer polarization (P). The tilt plane is perpendicular to P. Themolecular plane is tilted with respect to the layer normal. The shadingillustrates the orientation of the molecules. The stippled faces of theliquid crystals correspond to the portion of the molecule which is onthe outside of the curve of the molecule while the open faces of theliquid crystals correspond to the portion of the molecule which is onthe inside of the curve of the molecule. “Right” and “Left” designationsin FIGS. 3 and 4 are the chirality descriptors corresponding to right-and left-handed conformations. The single dashed lines representsynclinic interfaces in the anticlinic states. The double dashed linesrepresents defect walls separating oppositely tilted synclinic layers.For each of FIGS. 3A-4B, only two domains containing two layers each areshown for the sake of clarity of the figures. It should be understood,however, that each of the domains may have any number of layers and thatany number of domains may be present within a given liquid crystaldevice according to the present invention.

[0051] The layers shown in the figures are right- or left-handed,depending on the relative orientations of the two-fold symmetry axis andthe tilt direction. The term two-fold symmetry axis is the axis on whichthe molecule may be rotated 180° with no net change in the structure ofthe molecule. As mentioned above, the structure is called racemic if thechirality in adjacent layers within a domain alternates, and chiral ifthe adjacent layers within a domain have the same handedness.

[0052]FIGS. 3A and 3B show the layer and director structures of aracemic B2 banana phase. Most B2 phases have an antiferroelectric (AFE)ground state. In FIG. 3A, a synclinic tilted smectic (SmCS) polarantiferroelectric (PA) phase is shown. The phase is synclinic, whichmeans that the molecules in adjacent layers within the same domain tiltin the same direction, independent of the chirality of the phases. AnAFE state exists when a phase exhibits no net polarization direction. Inan AFE banana-shaped LC phase, the polarization director alternates 180°from one layer to the next. This arrangement can be seen in the tiltplane view of the two domains shown in FIG. 3A.

[0053] The textures of this phase usually consist of fan shaped domainswith stripes a few microns wide. Each stripe has a synclinic directortilt structure with a tilt angle

[0054] In the subsequent stripes, the tilt directions are in theopposite direction. The different tilt directions between one stripe andanother are represented by the top and bottom halves of FIG. 3A. Theoppositely tilting synclinic director structures are separated by defectwalls, represented by a double dashed line in FIG. 3A. These defectwalls typically have a defractive index different than that of theordered part of the material. The heterogeneity in the refractive indexfield can lead to a scattering of the unpolarized light, making thedevice opaque.

[0055] The AFE racemic state can be switched to a ferroelectric (FE)racemic state by the application of an electric field below 10 kHz. Aferroelectric state exists in a banana-shaped LC phase when there is anet polarization of a domain. In one embodiment, the change from an AFEto a FE state occurs at a field strength of about less than 10 V/μm. Ithas also been found that by applying a high frequency electric field,i.e. greater than about 10 kHz, the LC material switches from FE racemicto AFE racemic. By applying a low frequency electric field, i.e. below10 kHz, the LC material switches from AFE racemic to FE racemic. FIG. 3Bshows an anticlinic tilted smectic (SmC_(A)) polar ferroelectric (PF)phase. The term anticlinic refers to an opposite tilt of the moleculesin adjacent layers within a domain, as seen in the top and bottom partsof the tilt plane view. In this state, the optical axis is parallel tothe layer normal, independent of the sign of the external electricalfield. In the FE state, the defect walls of the AFE state are replacedby synclinic interfaces, which do not scatter light. Thus, a racemicstructure can be switched between a scattering or opaque “off” state byremoving an electric field to a transparent “on” state by applying anelectric field of sufficient magnitude, just as in polymer dispersedliquid crystals (PDLCs). As noted, switch can also be accomplished withchanges in frequency. In contrast to PDLCs, however, the switching timeof a racemic structure from an AFE to FE state is on the order ofapproximately 100 microseconds (μs) or less, which is more than an orderof magnitude faster than the switching time of PDLC devices.

[0056] In an FE state, the optical axis is parallel to the layer normal,independent of the sign of the external electrical field. Accordingly,no electro-optical switching is observed when a square wave field isapplied to phases in FE states. That is, there is no variation in theoptical modulation behavior of the LC when the electric field abruptlychanges from negative to positive, or vice versa, with constantamplitude.

[0057] The chiral B2 phase is shown in FIGS. 4A and 4B. In FIG. 4A, theanticlinic tilted smectic (SmC_(A)) polar antiferroelectric (P_(A))phase is shown. As described above, the term anticlinic refers to anopposite tilt of the molecules in adjacent layers within a domain, asseen in the top and bottom halves of the tilt plane view. The opticalaxis is again parallel to the layer normal regardless of the handednessof the phases. Layers with different handedness are separated by onlysynclinic interfaces in this phase. Therefore, this state is opticallyclear.

[0058] In the FE state of chiral domains, the director structure becomessynclinic. Therefore the phase is described as synclinic tilted smectic(SmC_(S)) polar ferroelectric (P_(F)). In this phase, the left andright-handed synclinic domains are separated by defect walls, whichscatter light. The tilt direction of this phase depends on the sign ofthe electric field. Therefore, electro-optical switching can be observedbetween crossed polarizers when a square wave electric field is appliedto a chiral B2 phase.

[0059] As with the above-described racemic phase, the chiral phase canbe induced to change from an AFE state to a FE state by the applicationof an electric field. Therefore, a chiral structure may also be switchedfrom a transparent state to an opaque state. As opposed to a racemicstate, however, a chiral state is transparent in the “off” state andscattering or opaque in the “on” state. The switching times between thechiral states are similar to the switching times of the racemic states.

[0060] In both the racemic and chiral states, the AFE state may beinduced to change to a FE state by the application of an electric field.During a field-induced AFE to FE transition, the layer chirality isassumed to be conserved. This assumption is generally true forshort-term application of the fields. In some cases, however, atransformation of the layer chirality can be observed. Stated anotherway, the SmC_(S)P_(A) arrangement shown in FIG. 3A can be transformedinto a SmC_(S)P_(F) shown in FIG. 4A. This transformation from an AFEracemic to an AFE chiral state corresponds to a transformation from ascattering to clear state. In some materials, the AFE chiral state canthen be transformed back to the AFE racemic state by application oftriangular-shaped electric fields. In such an electric field, the fieldchanges from positive to negative linearly, causing the LC material tobecome transiently antiferroelectric as the field passes through a zerofield state. In other words, the application of the triangular fieldwave form to an AFE chiral state transitions the material to a FE chiralstate then an FE racemic state and then to an AFE racemic state.Conversely, application of a square field wave form to an AFE racemicstate transitions the LC material to the FE racemic, to the FE chiral,and then to the AFE chiral state. Both the antiferroelectric chiral andantiferroelectric racemic states are stable at zero fields. In such anembodiment, a LC device using banana-shaped LCs can be used to form astable image without the continuous application of an electric field. Inother materials, the chiral state spontaneously relaxes back to theracemic state.

[0061] Advantageously, the scattering and transparent nature of thevarious states described above render them useful in electro-opticaldisplay devices. Specifically, the defect walls separating synclinicdomains, and their absence in the anticlinic domains, have importantconsequences for display applications, including faster switching times,larger viewing angles, and improved transmittance are possible. Further,the display modes can be interchanged.

[0062] Liquid crystal materials suitable for use in the methods anddevices of the present invention include liquid crystal materialscomprising banana-shaped molecules. As shown in FIG. 5, a lightmodulating device 10 comprises a pair of opposed substrates 12.Substrates 12 may be glass, plastic or other material commonly known inthe art. Transparent electrodes 14 may be disposed on substrates 12. Inone particular embodiment, electrodes 14 are indium-tin oxide. A powersource 16, is attached to electrodes 14 through a switch 18. The switch18 may be used to connect the power source to the electrodes, to shortthe electrodes, or to disconnect the electrodes to store charge on them.Operation of switch 18, may be controlled by an appropriately designedelectronic drive. Use of an electronic driver circuit allows particularareas of a matrix cell device to be addressed, which in turn allows highcontrast between the areas. As shown in FIG. 5, a banana-shaped LCmaterial is disposed between substrates 12 by any known method in theart, such as capillary action, for example. In FIG. 5, a SmC_(S)P_(F)arrangement is shown, although other arrangements may also be induced asdescribed herein.

[0063] In order to demonstrate the practice of the present invention,the following examples are presented. The specific materials used, thephase sequences of those materials from isotropic to liquid crystal tocrystalline phases and their phase transition temperatures are shown inTable 1. TABLE 1 Name Composition Phase sequence #1 53% B(4Cl)12 OO +47% I 130° C. B2 < 20° C. Cr 3FB10q #2 B14 I 153° C. B2 130° C. B3 91°C. Cr #3 B14 + 1.5% (1:1 ZLI 811/ZLI I 150° C. B2 127° C. B3 91° C. Cr3786) #4 B14 + 1.5% ZLI 811 I 150° C. B2 127° C. B3 91° C. Cr

[0064] The material B(4Cl) 1200 is 4-chloro-1,3-phenylenebis[4-(4-n-oxyphenylpropenoate) benzoate. B(4Cl) 1200 is represented byformula I,

[0065] R₁ is chlorine, R₂, R₃, and R₄ are hydrogen, and R₅ and R6 areC₁₂ alkoxy. Compound B(4Cl) 1200 forms a nematic phase in singlecompound form.

[0066] The material 3FB10q is 1,3-phenylenebis[3-fluoro-(4-n-decaoxyphenyliminomethyl)benzoate. 3FB10q isrepresented by formula I where

[0067] R₁ and R₂ are hydrogen, R₃, and R₄ are fluorine, and R₅ and R₆are C₁₀ alkoxy. The single compound forms a so-called B7 banana-phase.

[0068] The material B14 is 1,3-phenylenebis[4-4(4-tetradecoxyphenyliminomethyl)benzoate. B14 is represented byformula I where

[0069] R₁, R₂, R₃, and R₄ are hydrogen, and R₅ and R₆ are C₁₄ alkoxy.B14 will form a B2 phase by itself.

[0070] ZL1811 and ZL13786 are chiral dopants that are commerciallyavailable from Merck. The chemical structures of the two materials arethe same, but they are optical antipodes. ZLI 811 has (S) chirality,whereas ZLI 3786 has (R) chirality. Materials #3 and #4 have the samestructures, but #4 has chiral molecules, whereas #3 has only racemic.Material #1 has a B2 phase even at room temperature, and therefore isthe most useful for practical applications. A 1:1 mixture of materialsshown in FIG. 1A and FIG. 1B also forms a B2 phase at room temperatureand its performance is very similar to material #1. Material #2 was usedto illustrate that the disclosed electro-optical mode is not specificfor composition, but for the B2 phase. Materials #3 and #4 were used toillustrate that the underlying mechanisms require racemic molecules.

[0071] The studies were carried out in ready-made cells (4-μm cells fromDisplaytech, 5-μm and 10 μm thick cells from EHC). All cells disclosedwhich are described herein are opposed substrates, either glass orplastic, with electrodes disposed thereon. The cells were filled withthe aforementioned LC material and then sealed. Alignment properties maybe provided on the cells.

[0072] Upon cooling from the isotropic phase, samples #1 and #2 formed aracemic phase. The films were opaque because the textures containeddefect walls separating synclinic domains with opposite director tilt.The voltage dependence of the transmitted light intensity through sample#1 is shown in FIG. 6. Circles represent the data points for material #1in increasing fields, while squares represent the data points for thesame material #1 in decreasing fields.

[0073] The virgin cell is racemic and moderately scatters the light. Atincreasing electric fields at a frequency of 20 Hz, the transmittedlight intensity increases, especially where it switches to the FE stateat E˜8V/μm field. This behavior can be attributed to the disappearanceof the defect walls separating synclinic domains. At E˜10V/m thetransmittance decreases again, and the film becomes increasingly opaque.This is because the racemic structure becomes chiral. Although notwishing to condition patentability on any particularly theory, it isbelieved that such a transformation can be understood as the preferencefor synclinic interlayer interactions. The transformation to chiraldomains was simultaneously verified by studying the electro-opticalswitching under square wave electric fields with a polarizingmicroscope. At decreasing fields, the field dependence of thetransmittance is monotonous. Transmittance is low at high field, thanincreases as the texture switches back to the AFE state. In subsequentincreasing-decreasing field treatment the material stays in the chiralstate and the latter curve completely reproduces. In other words, in thechiral phase, the material can work as a stable electro-optical devicewithout hysteresis.

[0074] The transformation shown in FIG. 6 from the scattering racemicAFE state to the optically clear, stable, chiral AFE state by highfields illustrates the suitability of banana-shaped liquid crystals foruse in storage devices.

[0075]FIG. 6 also shows the increase in transmittance as thebanana-shaped liquid crystals go from the AFE racemic state to the FEracemic state. This characteristic makes them suitable for use indevices where PDLC-type switching devices were previously used. Itshould be noted that in the present example, the FE state does notbecome completely transparent, because of the eventual formation of thechiral state.

[0076] Another important feature of banana-shaped liquid crystals, shownin FIG. 6, is that the B2 phase can be switched between transparent andscattering states. It is remarkable that at high fields, more than 50%of the light is scattered out. This is similar to a reverse phase PDLC,which has about 60% turbidity, depending on the system.

[0077] The capability of using the banana-shaped liquid crystalsdisplays as light shutters is illustrated in FIGS. 7A and 7B. Behind acell filled with Material # 1, a sheet of paper is placed with the name“ALCOM” written on it. At zero field the film is transparent and thetext is visible as seen in FIG. 7A. At fields E>8V/μm, the film isopaque and the text is not visible (FIG. 7B). It is important to notethat the situation does not depend on the viewing angle. This is a majoradvantage over PDLCs.

[0078] The cells were also measured in reflection by microscope withoutpolarizers under laser illumination. The corresponding cells are shownin FIGS. 8A and 8B which are photomicrographs of 4 μm cells of material#1 at room temperature. FIG. 8A shows a border area of the cell at zerovoltage. FIG. 8B is the same cell with an applied voltage of 40V. Theimages represent 60 μm×60 μm areas.

[0079] The textures of cells containing banana-shaped LCs intransmission with white light illumination are shown in FIGS. 9 and 9B.FIG. 9 is a pair of photomicrographs showing textures of a 4 μm thickcell of material #1 in transmission mode without polarizers. Thetemperature was 23° C. The area shown is a 500 μm×300 μm area at theedge of the electrode. The photomicrograph on the left shows thetransmission properties of the cell with no electric field. Thephotomicrograph on the right shows the transmission properties of thecell with an electric field applied. FIG. 9B is a pair ofphotomicrographs of the cells shown in FIG. 9, except thephotomicrographs shown in FIG. 9B are at a higher magnification. Eachphotomicrograph shows a 50 μm×40 μm area of the cells.

[0080] The wavelength dependence of the scattering effect is shown inFIG. 10. The transmission spectra shown is for a 4 μm thick cell atmaterial #1 at 80° C.

[0081] The voltage and the temperature dependencies of the switchingtimes of a LC according to the present invention are shown in FIG. 11.FIG. 11A is a graph showing switching time versus voltage for a 4 μmcell containing material #1 at 75° C. FIG. 11A shows that the switchingtimes are generally 600 μs or less. When V/4 μm equals about 90, theswitching time is about 100 μs. FIG. 11B shows a graph showing switchingtime versus temperature for a 4 μm cell containing material #1 at avoltage of 70V. It is seen that even as far as 70° C. below the clearingpoint, the switching time is below 100-μs. This is more than an order ofmagnitude faster than the switching time of PDLC devices.

[0082] Similar results, with somewhat weaker scattering is observed incells containing materials #2 and #3. Transmission in the scatteringstate is 60% of the clear state. In addition, using these materials thechiral state can be transformed back to the racemic state by changingthe electric field waveform shape from square-wave to triangular. Thetransitions take about 1 second with a frequency of about 1 kHz.

[0083] The scattering almost disappears in cells containing material #4,which contains chiral dopant. The transmittances in OFF and ON statesdiffer only by 5%. This clearly proves that the scattering is connectedto the presence of left- and right-handed domains. The 1.5% chiraldopant makes the material almost completely uniformly chiral. Thedifferences between the racemic and chiral textures are presented inFIG. 12. In each of FIGS. 12A-12D, the figure is a photomicrograph ofthe texture of a 100-μm×70-μm area of a 4-μm cell between crossedpolarizers at a temperature of 123° C. FIG. 12A is a photomicrographicrepresentation of the texture of a of material B14 with 1.5% racemicdopant, at 0V. FIG. 12B is a photomicrograph of the texture of materialB14 with 1.5% racemic dopant at 30V. FIG. 12C is a photomicrograph ofthe texture of material B14 with 1.5% (S)enantiomer dopant, at 0V. FIG.12D is a photomicrograph of the texture of material B14 with 1.5%(S)-enantiomer dopant, at 30V. It can be seen that the racemic materialbreaks up into small domains in the ferroelectric state. The domains ofthe chiral materials are similar and substantially without defect linesin both the ferroelectric and the antiferroelectric state.

[0084] In one preferred embodiment of the present invention, a liquidcrystal cell is provided which contains banana-shaped liquid crystalmolecules in the racemic state. The switching takes place betweensynclinic and anticlinic structures at zero and sufficiently high(E>E_(th)) A.C. electric fields are applied. In appearance the film isopaque at zero field and clear under electric fields. The scattering atlow fields is due to the defects separating oppositely tilted synclinicdomains in the antiferroelectric state. Under sufficiently strong fieldsa ferroelectric state is induced where the defect walls disappear,because the oppositely tilted synclinic domains are anticlinic. Theclear state is clear in any direction. As the scattering is based on thetilt separation, it is called a tilt separation mode liquid crystaldevice (TSM-LCD).

[0085] In a second preferred embodiment, a cell is provided whichcontains banana-shaped liquid crystal molecules in the chiral state.Electro-optical switching takes place between anticlinic and synclinicstructures as zero and sufficiently high (E>E_(th)) A.C. electric fieldsapplied. In appearance the film is clear at zero field and opaque underelectric fields. At low fields the structure is antiferroelectric wherethere are no defect walls but only synclinic interfaces in an anticlinicbackground, therefore no light scattering appears. The scattering athigh fields is due to the defects separating oppositely tilted synclinicdomains in the ferroelectric state. Due to the overall racemic nature ofthe molecules the texture splits to left and right-handed synclinicdomains separated by walls that scatter the light. Because the lightscattering in this case is caused by defect walls that separate chiraldomains, it can be called a chiral separation mode liquid crystal device(CSM-LCD).

[0086] In a third preferred embodiment, a cell is provided that containsboth racemic and chiral banana-shaped structures. Application of asufficiently high electric field produces reversible transitions betweenthe racemic and chiral structures. Both states are stable at zerofields. The racemic state is scattering and is obtained by applicationof a triangular shape form and the chiral state is optically clear andis obtained by application of a rectangular shape form. These devicesare suitable for optical storage devices. As this method is based ontransitions between racemic and chiral states, it is called aracemic-chiral transitions mode liquid crystal device (RCT-LCD).

[0087] Although in appearance the disclosed methods and displays aresimilar to PDLC-s and PNLC-s, the underlying principles are completelydifferent. There are important differences in the performances of thedevices of the present invention and the PDLCs and PNLCs of the priorart. In PDLC-s and PNLC-s, scattering is due to heterogeneous materials.They involve the coexistence of solid and liquid crystal phases. In thepresent case, however, there is only one phase having either differentdirector tilt, or opposite chiral handedness.

[0088] In PDLC-s and PNLC-s the switching times are over onemillisecond, whereas in the devices of the present invention switchingtimes can be 10-μs or less. This is about two orders of magnitude fasterthan PDLCs and PNLCs. This is due to the polar nature of these phases,which provide first order interactions between field and polarization.In addition, the viewing angle and the transmittance of the clear stateare not limited when banana materials are used.

[0089] The liquid crystal devices of the present invention havecommercial application possibilities in all the areas where PDLC-s arecurrently used. This includes privacy windows, projectors, and the like.In addition, because the performance of the display devices of thepresent invention is superior in several aspects, including largerviewing angle and faster switching, the application possibilities arebroader. The fact that the racemic and chiral states work in oppositefashion and can be exchanged reversibly implies, for example, use in aprivacy window. Such a window does not use any energy, except duringswitching from one state to other. A RCT-LCD could also be used inelectronic newspapers, or in other optical data storage devices. Thetime for transformation from one state to the other requires about asecond, which is about the time to turn one page over in a book.Accordingly, they are completely satisfactory for these applications. Inaddition, a display can be switched to a mode, in which it stays in thechiral state and would be switched at a video rate for viewing motionpictures. This capability would make it useful in cellular phones,laptops or palmtops, etc. They also can be used in guest-host typedisplays with dichroic dyes. Furthermore, it is envisioned that theycould be used in one and two dimensional switchable gratings for beamsteering, and as optical switches for information technology. During thetransformation between racemic and chiral states, any state is stable,enabling multistable storage devices with gray scale properties. Grayscale may be achieved by varying the voltage magnitude or alternatively,by varying the frequency of the electric field.

[0090] Thus, it can be seen that the objects of the invention have beensatisfied by the structure and its method for use presented above. Whilein accordance with the Patent Statutes, only the best mode and preferredembodiment has been presented and described in detail, it is to beunderstood that the invention is not limited thereto or thereby.Accordingly, for an appreciation of true scope and breadth of theinvention, reference should be made to the following claims.

What is claimed is:
 1. A liquid crystal device comprising tilted smecticphases of banana-shaped liquid crystal molecules.
 2. The liquid crystaldevice according to claim 1, wherein the device is an electro-opticalstorage device.
 3. The liquid crystal device according to claim 1,wherein the device is an electro-optical switching device.
 4. The liquidcrystal device according to claim 1, additionally comprising: a pair ofopposed substrates, each substrate having an electrode facing the othersubstrate; and wherein said tilted smectic phases of banana-shapedliquid crystal molecules are disposed between said pair of substrates ina racemic state.
 5. The liquid crystal device according to claim 4,wherein said banana-shaped liquid crystal molecules are synclinic belowa threshold voltage applied across said electrodes and anticlinic abovesaid threshold voltage.
 6. The liquid crystal device according to claim1, additionally comprising: a pair of opposed substrates, each substratehaving an electrode facing the other substrate; and wherein said tiltedsmectic phases of banana-shaped liquid crystal molecules are disposedbetween said pair of substrates in a chiral state.
 7. The liquid crystaldevice according to claim 6, wherein said banana-shaped molecules areanticlinic below a threshold voltage applied across said electrodes andsynclinic above said threshold voltage.
 8. The liquid crystal deviceaccording to claim 1, additionally comprising: a pair of opposedsubstrates, each substrate having an electrode facing the othersubstrate; and wherein said tilted smectic phases of banana-shapedliquid crystal are disposed between said pair of substrates in both aracemic and a chiral state.
 9. The liquid crystal device according toclaim 8, wherein application of a sufficiently high electric fieldtransitions said material between the racemic and chiral states, andwherein both racemic and chiral states are stable in the absence of anelectric field.
 10. The liquid crystal device according to claim 8,wherein said material exhibits gray scale properties that are stable inthe transition between the racemic and chiral states.
 11. The liquidcrystal device according to claim 8, wherein the switching time betweenthe racemic and chiral states is less than about 100 microseconds. 12.The liquid crystal device according to claim 1, wherein thebanana-shaped liquid crystal molecules are selected from the groupconsisting of compounds represented by formula I

R₁, R₂, R₃, R₄ are independently hydrogen or a halogen, and R₅ and R₆are independently C₈-C₁₆ alkyl or C₈-C₁₆ alkoxy.
 13. The liquid crystaldevice according to claim 1, wherein the banana-shaped liquid crystalmolecules are selected from4-chloro-1,3-phenylinabis-1,3-phenylenebis[4-(4-14alkyloxyphenyliminomethyl)benzoate, 1,3-phenylenebis[4-4(4-n-alkylphenyliminomethyl)benzoates, 1,3-phenylenebis[4-4(4-n-alkyloxyphenyliminomethyl)benzoates, 1,3-phenylenebis[3-fluoro-(4-n-alkyloxyphenyliminomethyl)benzoates, and halogenatedderivatives thereof.
 14. A method for fabricating a light modulatingdevice, the method comprising the steps of: providing a pair ofsubstrates with a cell gap therebetween; and permanently disposing atleast one banana-shaped liquid crystal material into said cell gap. 15.The method for fabricating a light modulating device according to claim14, wherein transparent electrodes are disposed on each of saidsubstrates adjacent said cell gap.
 16. The method for fabricating alight modulating device according to claim 14, 2 wherein the at leastone banana-shaped liquid crystal material is selected from 3 the groupconsisting of compounds represented by formula I

R₁, R₂, R₃, R₄ are independently hydrogen or a halogen, and R₅ and R₆are independently C₈-C₁₆ alkyl or C₈-C₁₆ alkoxy
 17. The method forfabricating a light modulating device according to claim 14, wherein theat least one banana-shaped liquid crystal material is selected from thegroup consisting of 4-chloro-1,3-phenylinabis-1,3-phenylenebis[4-(4-14alkyloxyphenyliminomethyl)benzoate, 1,3-phenylenebis[4-4(4-n-alkylphenyliminomethyl)benzoates, 1,3-phenylenebis[4-4(4-n-alkyloxyphenyliminomethyl)benzoates, 1,3-phenylenebis[3-fluoro-(4-n-alkyloxyphenyliminomethyl)benzoates, and halogenatedderivatives thereof.
 18. A method of generating an image, comprising:providing a pair of substrates with a cell gap therebetween; providingtransparent electrodes on each of said substrates adjacent to said cellgap; permanently disposing at least one banana-shaped liquid crystalmaterial into said cell gap; and applying an electric field across saidelectrodes.
 19. The method according to claim 18, wherein said step ofapplying an electric field further comprises: applying an electric fieldcapable of selectively causing said banana-shaped liquid crystalmaterial to change between an antiferroelectric state and aferroelectric state.
 20. The method according to claim 18, wherein saidstep of applying an electric field further comprises: applying anelectric field capable of selectively causing said banana-shaped liquidcrystal material to change between a racemic phase and a chiral phase.21. The method according to claim 18, wherein said step of applying anelectric field further comprises: applying an electric field capable ofselectively causing said banana-shaped liquid crystal material to changefrom a chiral phase to a racemic phase.